Historically programs added /usr/lib/mysql to the library search path when
linking. mariadb does not do so anymore, although even its mysql_config
script suggests to use this path. When an old version of the library still
resides in this directory (like from an old mysql installation) it will be
linked against instead of the current library which is in /usr/lib.

To prevent this simply install symlinks from the staging libs in /usr/lib
into /usr/lib/mysql. This fixes build failures on the build bots which are
seen with libzdb and libdbi-drivers.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

Observed once in about two dozen builds on the bots:

make[6]: *** No rule to make target '/build/lede-snapshots/arm_cortex-a7_neon-vfpv4/build/sdk/staging_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/usr/lib/libevent.so', needed by 'tests/async_queries'.  Stop.

Address this by adding libevent2 to PKG_BUILD_DEPENDS.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

MariaDB is a drop-in replacement for MySQL. This commit adds a current
and stable version of MariaDB to the tree.

Quite a few ideas/patches were copied from Alpine Linux, Busybox Buildroot
and Debian.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>